1. Field of the Invention
The present invention relates to an image forming method and apparatus capable of forming images at ultra-high speeds, and a toner applicable to the image forming method and apparatus.
2. Discussion of the Background
Demands for high-speed printing and high quality images are increasing in the electrophotographic industry recently. Generally speaking, however, the higher the image forming speed (hereinafter “system speed”) of an image forming apparatus, the worse the resultant image quality. Specifically, fixability of a toner deteriorates as the system speed increases, thereby degrading the resultant image quality. For this reason, a technique for providing both high-speed printing and good fixing performance is needed.
A toner image is typically fixed on a recording medium, such as paper, by application of heat and pressure in a fixing device. As the system speed increases, the toner image receives less heat in the fixing device. As a result, the toner may not be fixed on the recording medium and may release therefrom.
To prevent the deterioration of the image quality even when the system speed increases, one proposed approach involves increasing a fixing temperature. However, this approach has a possibility of causing side effects of temperature rise inside an image forming apparatus, short life of a fixing member, and consumption of a large amount of energy. Therefore, a toner itself is required to have improved fixability especially for use in ultrahigh-speed machines. Specifically, a toner is needed that is capable of providing good fixability with less heat even when used in a high-speed fixing device.
Various attempts have been made to improve fixability of a toner. For example, one proposed approach involves controlling thermal properties of a binder resin. However, if the glass transition temperature (Tg) of a binder resin is reduced, thermostable preservability and fixing strength of the resultant toner may deteriorate. Alternatively, if the molecular weight of a binder resin is reduced, a hot offset problem may occur and the resultant image may be too glossy. The “hot offset” here refers to an undesirable phenomenon in which part of a fused toner image is adhered to the surface of a heat member, and re-transferred onto an undesired portion of a recording medium. Consequently, a toner having a good combination of fixability and thermostable preservability is not yet provided only by controlling thermal properties of binder resins.
Accordingly, toners using polyester resins, which are fixable at lower temperatures and have better thermostable preservability than conventionally-used styrene-acrylic resins, are disclosed in Unexamined Japanese Patent Applications Publications Nos. (hereinafter “JP-A”) 60-90344, 64-15755, 02-82267, 03-229264, 03-41470, and 11-305486. In addition, JP-A 62-63940 discloses a toner including a binder resin including a non-polyolefin crystalline polymer so as to improve fixing ability at low temperatures (hereinafter “low-temperature fixability”). However, the molecular structure and molecular weight of the non-polyolefin crystalline polymer is not optimized therein.
Japanese Patent No. (hereinafter “JP”) 2931899 and JP-A 2001-222138 each disclose a toner including a crystalline polyester having a quickly-melting property, as well as the above-described non-polyolefin crystalline polymer, with an intention of improving fixability of the toner. However, the toner of JP 2931899 has poor affinity for paper because of having too low an acid value of 5 or less and too low a hydroxyl value of 20 or less, resulting in poor low-temperature fixability. Further, the molecular structure and molecular weight of the crystalline polymer in not optimized in JP-A 2001-222138. Such a toner may not provide low-temperature fixability either when an oil is applied to a fixing roller or not. Consequently, such a toner may not provide a good combination of low-temperature fixability, hot offset resistance, thermostable preservability, transferability, durability, charge stability in humid conditions, and pulverization efficiency.
Further, JP-A 2002-214833 discloses a toner including a crystalline polyester and an amorphous crystalline polyester having no affinity with each other, forming a sea-island phase-separated structure. A DSC curve of THF-soluble components of the toner has a maximum peak at a specific temperature, so that the toner has both low-temperature fixability and thermostable preservability. However, such a toner does not solve the problems of inconsistent image density and image defects caused by an undesired toner film formed on an image bearing member.
JP-A 2005-338814 also discloses a toner including a crystalline polyester resin. However, this toner may form an undesirable toner film on image forming components, resulting in unreliable image quality.
As described above, a crystalline polyester resin improves low-temperature fixability of a toner. However, a toner including a crystalline polyester resin has a drawback that the toner tends to form undesirable films thereof on the surface of an image bearing member (hereinafter “filming problem”), thereby causing image defects, specifically, white spots in the resultant image.
It is considered that the filming problem is caused due to the nature of a crystalline polyester resin. More specifically, it is considered that the crystalline polyester resin exudes to the surface of the toner, and then contaminates an image bearing member, resulting in occurrence of the filming problem.
A wax and other external additives also have an influence on low-temperature fixability and the occurrence of the filming problem.
The presence of a wax improves low-temperature fixability of a toner. Further, a wax is considered to promote a crystalline polyester resin to exude from the toner. Therefore, as the amount of a wax included in a toner increases, the filming problem may more easily occur.
On the other hand, an external additive has a function of scraping off undesirable toner films formed on an image bearing member. Therefore, as the amount of an external additive added to a toner increases, the filming problem may occur much less. However, as the amount of an external additive added to a toner increases, the void ratio of an unfixed toner image increases, thereby degrading thermal conductivity of the unfixed toner image when fixed on a recording medium. Further, too large an amount of an external additive may disturb fixation of an unfixed toner image.